Processes utilizing corrosive Friedel-Crafts type catalysts for the alkylation of aromatic hydrocarbons have long been known, and are still widely practiced commercially. It has been known for some time that the principal disadvantages of such processes can be avoided by the use of acidic, crystalline aluminosilicate zeolite catalysts, which are non-corrosive and from which the alkylation products can be more readily separated.
In using solid zeolite catalysts, two principal modes of operation have been described. Firstly, the catalyst may be utilized as a powder slurried in the liquid reactants. This procedure is disadvantageous because it generally requires batch as opposed to continuous operation, and also requires expensive filtration or centrifuging units to separate catalyst from products. A much more commercially feasible technique involves the use of a fixed bed of relatively large catalyst particles through which the reactants are continuously passed. A first practical requirement in such fixed bed operations is that the catalyst particles must be sufficiently large to permit passage of reactants through the bed without developing a prohibitive pressure drop therethrough. This requirement can in turn place limitations on the efficiency of utilization of active sites located in the interior of the catalyst particles, which are accessible only by diffusion through macropores in the outer layers of the catalyst particles.
In certain fixed-bed processes utilizing catalysts containing an active hydrogenating metal and carried out in the presence of high pressure hydrogen, it has been disclosed that pressure drop can be minimized while obtaining increased bulk-volume catalyst activity by forming the catalyst particles into various non-cylindrical shapes which exhibit an increased ratio of exterior surface area to volume, and which form packed beds having increased void space. These processes include catalytic hydrofining of mineral oils to remove sulfur and nitrogen compounds (U.S. Pat. Nos. 3,674,680, 3,990,964 and 4,028,227) and catalytic hydroforming of naphthas (U.S. Pat. No. 3,857,780). Typical catalyst particles described are extrudates whose cross sections embrace a plurality of arcuate lobes extending outwardly from the central portion thereof in cloverleaf fashion.
Although an increase in the exterior surface area/volume ratio of porous catalyst particles (as by decreasing size or modifying shape) might be expected to give some increase in fresh activity, it does not follow that such increased activity would be of a significant magnitude in all cases, or that a concomitant increase in deactivation rate might not completely outweigh any improvement in fresh activity. In most hydrocarbon conversion processes, including alkylation, the principal catalyst-deactivating factor resides in the deposition of polymers, or coke-like materials on the active catalyst surfaces. In cases such as those discussed above, wherein hydrogen and a catalytic metal are present, the formation of polymers and coke can be suppressed almost to an equilibrium value by hydrogenation of coke and polymer precursors. However, in alkylation with olefins as herein, hydrogen is necessarily absent, and the catalyst contains no hydrogenating metal. In our initial investigation of alkylation with zeolite catalysts, we found that catalyst deactivation rates were a much more serious problem than was initial activity. The zeolite catalysts contain no known polymerization-suppressing component; their acid function promotes both alkylation and polymer formation. It was hence uncertain as to whether initial activity could be improved without aggravating the more serious problem of catalyst deactivation rates.
This deactivation problem is well-recognized in the art, and it has been fairly well established that the mechanism thereof involves polymerization of the olefin, followed by hydrogen-transfer and cyclization reactions to form large aromatic molecules which cannot diffuse out of the crystal micropores of the zeolite in which the active sites are located. (Venuto et al, J. Catalysis 5, 484-493, 1966; I and EC Product Research and Development, 6, 190-192, Sept. 1967). The rate of such deactivation is in direct proportion to the concentration of olefin in the interior of the catalyst particles. In order to minimize olefin polymerization and polyalkylation of the aromatic hydrocarbon, it is customary in such alkylations to utilize a feed mixture comprising about 4-10 moles of aromatic hydrocarbon per mole of olefin. When using catalyst particles of the usual commercial size and shape, e.g. 1/16" cylindrical extrudates, the diffusion of such feed mixtures into each catalyst particle via its interstitial macropore structure, and the resultant depletion of olefin by alkylation, establishes a concentration gradient of olefin in each particle--high near the exterior surface thereof and low in the interior. This would be expected to establish a zone of rapid deactivation in the outer shell of the particle, which zone would gradually shift toward the interior, assuming no substantial blockage of the interstitial macropores.
In view of all the foregoing, it would appear that any reduction in particle size, or change in shape to give a lower ratio of volume to exterior surface area, would result in an increase in average olefin concentration therein (by reducing the olefin concentration gradient). This in turn would be expected to act in the direction of increasing the deactivation rate of each particle, and this would not be expected to increase the cycle life of a bed of catalyst particles. We have discovered however that a substantial such increase is obtained; apparently some other operative factor is brought into play when the volume/surface area ratio is decreased which actually leads to a substantial decrease in deactivation rate and an increase in cycle life. At the same time there is a significant increase in bulk volume activity and selectivity of conversion to monoalkylated products.